Optical printers and scanners typically incorporate a mechanism to direct a light beam in a pattern that scans across the surface of an image plane surface. Often the light beam is a laser beam. For printers, the image plane surface is typically the outer cylindrical surface of a photo-sensitive drum. For scanners, the image plane surface is typically a piece of paper being scanned. Generally the light beam sweeps in one plane, called the horizontal plane, while the image plane moves in an orthogonal (vertical) plane. In cases where the image plane surface is a photo-sensitive drum, movement in the vertical plane is achieved by rotating the drum around its cylindrical axis.
In many optical systems the horizontal scanning motion of the light beam is achieved by reflecting the light beam off a moving mirror. Often the mirror is a series of planar surfaces on a polygonal wheel that rotates. In such systems, the light beam is focused at an oblique angle toward the axis of rotation of the mirror, and the light beam is deflected in a linear scanning mode by each planar mirror surface of the polygon as the mirror spins. In systems where the image plane surface is simultaneously moving in the vertical plane, the horizontal light scanning plane is tilted somewhat from the vertical recording medium plane such that the trace of the light scan is substantially orthogonal to the direction of movement of the of the image plane surface in the vertical plane.
In order to write information as in a printer, the ray of the light beam is typically modulated as it scans. In order to read information as in a scanner, the intensity of the reflected beam is typically monitored. To achieve an accurate recording of the image as successive scan lines are written on or read from the image plane surface, it is essential to synchronize the modulation of each scan of light beam with the modulation of the previous scans that are creating the image. Typically this synchronization is achieved by optically intercepting the light beam with one or more photo detectors at the start of each scan line. The detection of light beam by the photo detector creates a start of scan pulse that is used to synchronize the start of modulation of the light beam for each successive scan line.
Historically, in many optical scanning and printing systems, scanning occurs in only one direction. As a result, it has generally been sufficient to intercept the light beam on one side (the starting side) of each successive scan. However, optical printer and scanner users are continually demanding increases in processing speed. As speeds increase accurate synchronization of modulation (for printers) or detection (for scanners) of the light beam becomes more difficult using only a start-of-scan pulse. Also, in many applications it is desirable to provide bi-directional light beam scanning capability. Existing technology does not adequately address all the needs for synchronizing light beam modulation for high speed or bi-directional scanning or printing. What is needed is an improved means that has the ability to detect both the start and the end of each scan of the light beam. Also needed is an improved means of synchronizing bi-directional scanning systems.